1. Field of the Invention
The present invention relates generally to communication systems having receivers for receiving signals of one or more communication channels and particularly to a timing recovery device used in the receiver of a communication system causing ‘whitening’ to reduce the effects of fading in communication channels.
2. Description of the Prior Art
Timing recovery is a vital part of a communication system because it is the process of synchronizing a local receiver to a remote transmitter. Linear predictors have been used in communication systems due to their ability to pre-whiten a signal before blind adaptive equalization. That is, equalization of a communication channel, carrying a signal to be received and demodulated, is improved in a receiver using a linear predictor. An example of such use for the case of Constant Modulus Algorithm (CMA) blind adaptation is described in the paper “Blind adapted, pre-whitened constant modulus algorithm,” by James P. LeBlanc and Inbar Fijalkow, presented at IEEE International Conference on Communications, 2001, and incorporated here by reference. Systems that use linear prediction in the context of blind adaptive equalization are disclosed in U.S. Pat. No. 5,909,466 to Labat et. al., and U.S. Pat. No. 7,027,500 to Casas et. al.
Linear prediction has also been proposed in its recursive lattice configuration for use in a timing recovery system for magnetic recording. This application is described in “Recursive Linear Prediction for Clock Synchronization,” by M. U. Larimore and B. J. Langland, presented at IEEE International Conference on Acoustics, Speech, and Signal Processing, April, 1981, and incorporated here by reference.
Timing recovery is accomplished by analog, digital, or mixed analog and digital means. A conventional digital timing recovery architecture, as illustrated in FIG. 1a, includes a timing recovery module 100 including an A/D converter 106, a fixed rate sampling clock 107, a timing correction module 101 and a timing update module 104. The A/D converter 106 samples a received signal, carried by a communication channel, at a fixed rate determined by the fixed rate sampling clock 107. The sampling rate is not synchronized to a remote transmitter but such synchronization is necessitated, otherwise, the transmitted signal cannot be accurately recovered. The timing correction module 101 re-samples the signal at a rate synchronized to a remote transmitter. The synchronized signal is processed by the timing update module 104, which updates one or more parameters in the timing correction module 101 in order to maintain synchronization.
The output of the timing correction module 101 is a synchronous sampled signal whose sampling rate is synchronized to a remote transmitter. In general, the timing correction module 101 uses interpolation techniques, known to those skilled in the art, to generate the synchronous sampled signal. The publication “Interpolation in Digital Modems-Part I: Fundamentals,” by Floyd M. Gardner, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 41, NO. 3, MARCH 1993, provides more information about this architecture. The publication “Design of Optimal Interpolation Filter for Symbol Timing Recovery,” by Daeyoung Kim, Madihally J. Narasimha, and Donald C. Cox, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 45, NO. 7, JULY 1997, discusses the interpolation filter design in particular. Both publications are incorporated herein by reference.
A conventional mixed analog and digital timing recovery architecture, as illustrated in FIG. 1b, includes a timing recovery module 100 including an A/D converter 106, an adjustable rate sampling clock 108, and a timing update module 104. The A/D converter 106 samples a signal carried by a communication channel at an adjustable rate determined by the adjustable rate sampling clock 108. The sampled signal is processed by the timing update module 104, which adjusts the adjustable rate sampling clock 108 in order to maintain synchronization.
The timing update module 104 of FIG. 1a or FIG. 1b may use blind or decision-directed techniques to update one or more parameters in the timing correction module 101 of FIG. 1a, or to adjust the adjustable rate sampling clock 108 of FIG. 1b. Blind techniques are often preferred because they avoid the long delay of an equalizer in the timing loop, and they can be employed when reliable decisions are unavailable. Conventional blind techniques rely on excess bandwidth (signal energy above the Nyquist band-edges) to extract timing error information. These will be referred to hereafter as “band-edge timing recovery” techniques. One such technique is described in the publication “Passband Timing Recovery in an All-Digital Modem Receiver” by Dominique N. Godard, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 26, NO. 5, MAY 1978.
It is worthy to note that band-edge timing recovery techniques must operate at a sampling rate higher than the Nyquist rate of the transmitted signal in order to retain excess bandwidth information. More specifically, band-edge timing recovery requires higher than Nyquist sampling rate. Therefore, all signal processing operations in the timing loop have to operate at a rate higher than Nyquist sampling rate. This is in contrast to a conventional linear predictor used for blind equalization, which can operate at the Nyquist sampling rate (symbol rate.)
In a receiver, the output of the timing recovery module 100 is typically connected to an equalizer module (not shown in FIG. 1a or 1b). The equalizer may operate at the Nyquist rate, in which case it is known as a symbol-spaced or T-spaced equalizer. In that case, the signal must be down-sampled after timing recovery, and the timing update module 104 must not only synchronize the local receiver's sampling rate to a remote transmitter, it must also select the optimum down-sampling phase. If the wrong phase is chosen, symbol-spaced equalizer performance can be significantly degraded. The issue can be avoided by operating the equalizer at a rate higher than Nyquist (also known as fractionally-spaced equalization), but this increases the size and cost of the receiver.
In most practical communication systems, the transmitted signal is distorted by a communication channel before reaching the receiver. For example, in terrestrial digital television, the transmitted signal may reach the receiver via several different paths. This is known as “multipath distortion.” In some cases, the communication channel may severely attenuate the signal frequency components near its Nyquist band-edges thereby degrading the performance of band-edge timing recovery techniques.
Therefore, the need arises for an optimized timing recovery (or correction) device and method that improves the performance of band-edge timing recovery techniques when the Nyquist band-edges of the signal are degraded by the communication channel.